The invention relates generally to the detection and identification of airborne substances and more particularly to capturing aerosols and vapors directly on a substrate or medium for analysis using paper spray mass spectrometry.
Aerosols are colloidal suspensions of fine liquid droplets or solid particulates that are dispersed in air or gas. The particles or droplets can stay airborne for a long time and tend to move in accordance with the direction of air flow. Examples of natural and artificial aerosols include but are not limited to clouds, fog, dust, smoke, diesel exhaust, pollen and particulate air pollutants. Vapors are molecules of various chemicals in the gas phase. A vapor will stay suspended in air indefinitely unless removed, condensed, or trapped. As opposed to aerosols, vapors mostly consist of single, individual molecules moving randomly.
There is a growing need for improved detection, analysis, and identification of aerosols and vapors from a variety of sources. One example is the analysis of aerosolized chemical warfare agents (CWAs), such as those generated by an explosively dispersed chemical munition. Another example is airborne materials present in the environment including both chemicals and molecules of biological origin. There is also increased interest in capturing and identifying biomarkers or drugs of interest that may be present in human exhaled breath, which contains both aerosols and vapors.
Detection of aerosols and vapors can be done using many different technologies. Some of these technologies are primarily directed at the characterization of an aerosol itself such as droplet size, shape, and distribution. Other technologies, such as Light Detection and Ranging (LIDAR), are effective at detecting and tracking large aerosol clouds. However, these technologies typically lack the ability to specifically identify the chemical composition or the concentration of the aerosol or vapor.
In the case of CWAs, detection methods in battlefield and forensic situations can generally be categorized into either onsite direct analysis or capture for later analysis methods. Most onsite or field systems for the detection of aerosolized CWAs prioritize portability and fast analysis times. Examples of this type of system include an IMS-based (ion mobility spectrometry) sensor or colorimetric papers. Although simple to use, IMS sensor devices are unable to directly analyze aerosols. They also have other disadvantages, including sensitivity to temperature and humidity, false alarms due to lack of specificity, and detector saturation from exposure to high concentrations. Another method used in the field, U.S. Army M9 colorimetric papers, only react with a specific class of CWA and tend to have a high false positive rate. Additionally, the analysis of these papers relies on either human visual processing or optical sensors, both of which can be affected by differences in ambient light, color perception, or color blindness. Both of these systems only give qualitative results and are less effective if not completely ineffective at directly identifying or quantifying compounds in aerosolized form.
In contrast to these onsite direct analysis techniques, several approaches have been used to capture an aerosol or vapor for analysis at a later time. These include a variety of filter and sorbent based approaches. After exposure, the filter or sorbent is sent back to an analytical laboratory for processing. This processing typically involves at least a multi-step process of extracting the analyte using either heat or solvents and then identifying a compound in the analyte. The handling process introduces a delay in identification and can also introduce error due to improper handling. Frequently, these capture devices require chromatographic separations, extending analysis time and reducing overall throughput.
Ambient ionization is an analysis technique that uses mass spectrometry without sample preparation or separation. Paper spray mass spectrometry (PS-MS) is an ambient ionization approach in which samples are deposited on a triangular substrate. The substrate is wetted with a solvent, which often includes a charge carrier. A voltage is applied to the substrate, causing a spray of solvent and analyte molecules to emit from a tip of the triangular substrate for analysis. PS-MS has been shown to be highly effective for the detection and quantitation of pharmaceutical drugs, illicit chemicals (both illegal drugs and explosives) and analysis of samples from complex matrices including surfaces, food, blood, urine and tissue. Samples are collected or deposited directly onto a paper substrate from biological and environmental sources and analyzed by MS without the need for desorption or extraction. While PS allows for direct sampling with little to no sample preparation and rapid MS analysis, it has not been used to sample and detect aerosols and vapors.
Thus, a need exists for a system and method to capture aerosols and vapors from a large variety of sources, both anthropogenic and environmental, that requires less sample handling, processing, and extraction prior to analysis by PS-MS. There is a further need for a systematic method that has improved direct sampling efficiency and lower limits of detection by improving the collection of aerosols and vapors, while also maintaining compatibility with analytical techniques including but not limited to mass spectrometry. There is also a need for a system and method for aerosol and vapor collection that is compact and compatible with hand held devices and unmanned vehicles.